routing(4) routing(4)
NAME
routing - system support for packet network routing
DESCRIPTION
The network facilities provide general packet routing.
Routing table maintenance may be implemented in applications
processes.
Files
/dev/kmem
Diagnostics
EEXIST A request was made to duplicate an
existing entry.
ESRCH A request was made to delete a non-
existent entry.
ENOBUFS Insufficient resources were available to
install a new route.
USAGE
A simple set of data structures compose a ``routing table''
used in selecting the appropriate network interface when
transmitting packets. This table contains a single entry for
each route to a specific network or host. The routing table
was designed to support routing for the Internet Protocol
(IP), but its implementation is protocol independent and thus
it may serve other protocols as well. User programs may
manipulate this data base with the aid of two ioctl(2)
commands, SIOCADDRT and SIOCDELRT. These commands allow the
addition and deletion of a single routing table entry,
respectively. Routing table manipulations may only be carried
out by privileged user.
A routing table entry has the following form, as defined in
/usr/include/net/route.h:
struct rtentry {
u_long rt_hash; /* to speed lookups */
struct sockaddr rt_dst;/* key */
struct sockaddr rt_gateway;/* value */
short rt_flags; /* up/down?, host/net */
short rt_refcnt; /* # held references */
u_long rt_use; /* raw # packets forwarded */
#ifdef STRNET
Copyright 1994 Novell, Inc. Page 1
routing(4) routing(4)
struct ip_provider *rt_prov;/* the answer: provider to use */
#else
struct ifnet *rt_ifp;/* the answer: interface to use */
#endif /* STRNET */
};
with rt_flags defined from:
#define RTF_UP 0x1/* route usable */
#define RTF_GATEWAY 0x2/* destination is a gateway */
#define RTF_HOST 0x4/* host entry (net otherwise) */
Routing table entries come in three flavors: for a specific
host, for all hosts on a specific network, for any destination
not matched by entries of the first two types (a wildcard
route). Each network interface installs a routing table entry
when it is initialized. Normally the interface specifies the
route through it is a ``direct'' connection to the destination
host or network. If the route is direct, the transport layer
of a protocol family usually requests the packet be sent to
the same host specified in the packet. Otherwise, the
interface may be requested to address the packet to an entity
different from the eventual recipient (that is, the packet is
forwarded).
Routing table entries installed by a user process may not
specify the hash, reference count, use, or interface fields;
these are filled in by the routing routines. If a route is in
use when it is deleted (rt_refcnt is non-zero), the resources
associated with it will not be reclaimed until all references
to it are removed.
User processes read the routing tables through the /dev/kmem
device.
The rt_use field contains the number of packets sent along the
route. This value is used to select among multiple routes to
the same destination. When multiple routes to the same
destination exist, the least used route is selected.
A wildcard routing entry is specified with a zero destination
address value. Wildcard routes are used only when the system
fails to find a route to the destination host and network.
The combination of wildcard routes and routing redirects can
provide an economical mechanism for routing traffic.
Copyright 1994 Novell, Inc. Page 2
routing(4) routing(4)
REFERENCES
ioctl(2), route(1M), routed(1M)
Copyright 1994 Novell, Inc. Page 3